U.S. patent number 6,391,931 [Application Number 09/301,647] was granted by the patent office on 2002-05-21 for uniform small cell foams and a continuous process for making same.
Invention is credited to Bonnie Weiskopf Albrecht, Mark David Gehlsen, Craig Allen Perman, David Loren Vall.
United States Patent |
6,391,931 |
Gehlsen , et al. |
May 21, 2002 |
**Please see images for:
( Certificate of Correction ) ** |
Uniform small cell foams and a continuous process for making
same
Abstract
A continuous method for making foams having uniform and/or small
cell sizes and articles made with these foams are described. The
method allows for adjusting or controlling cell size and cell size
distribution by controlling temperature and/or blowing agent
concentration. The foams feature small and/or uniform cell sizes
and may be comprised of amorphous thermoplastic polymers, pressure
sensitive adhesive compositions, and immiscible thermoplastic
polymer compositions. A method for coextruding the foams is also
described.
Inventors: |
Gehlsen; Mark David (St. Paul,
MN), Vall; David Loren (St. Paul, MN), Perman; Craig
Allen (St. Paul, MN), Albrecht; Bonnie Weiskopf (St.
Paul, MN) |
Family
ID: |
23164264 |
Appl.
No.: |
09/301,647 |
Filed: |
April 28, 1999 |
Current U.S.
Class: |
521/50; 521/134;
521/79; 521/81 |
Current CPC
Class: |
B29C
44/348 (20130101); B29C 48/92 (20190201); B29C
48/05 (20190201); B29C 48/04 (20190201); B29C
48/385 (20190201); B29C 48/625 (20190201); B29C
48/9185 (20190201); B29C 48/919 (20190201); B29C
44/352 (20130101); Y10T 428/249978 (20150401); B29C
2948/92704 (20190201); B29C 2948/92514 (20190201); B29C
2948/926 (20190201); B29C 2948/92895 (20190201); B29C
2948/92904 (20190201); Y10T 428/24998 (20150401); Y10T
428/249986 (20150401); Y10T 428/249984 (20150401); Y10T
428/249953 (20150401); Y10T 428/249983 (20150401); Y10T
428/249979 (20150401) |
Current International
Class: |
B29C
44/34 (20060101); C08J 009/00 () |
Field of
Search: |
;521/50,134,135,136,137,138,139,140,79,81 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0 610 953 |
|
Aug 1994 |
|
EP |
|
0 707 935 |
|
Apr 1996 |
|
EP |
|
0 818 292 |
|
Jan 1998 |
|
EP |
|
WO 97/06935 |
|
Feb 1997 |
|
WO |
|
WO 98/08667 |
|
Mar 1998 |
|
WO |
|
Other References
Behravesh, Amir H.; Park, Chul B., and Venter, Ronald D.; Challenge
to the Production Of Low-Density, Fine-Cell HDPE Foams Using
CO.sub.2 ; Cellular Polymers, vol. 17, No. 5, Jan. 1, 1998; pp.
309-326..
|
Primary Examiner: Seidleck; James J.
Assistant Examiner: Bagwell-Bissett; Melanie
Attorney, Agent or Firm: Zillig; Kimberly S.
Claims
What is claimed is:
1. A foam having cell sizes of 5 to 50 micrometers, and cell
distribution polydispersities of less than 1.10.
2. The foam of claim 1 further comprising a semi-crystalline
material in an amount of less than 50 volume percent.
3. The foam of claim 1 having a dielectric strength greater than
300.
4. An article comprising one or more layers of material wherein at
least one layer comprises the foam material of claim 1.
5. A foam having cell sizes of 5 to 50 micrometers, cell
distribution polydispersities of less than 1.10, and foam densities
of 0.1 to 0.3 grams per cubic centimeter.
Description
FIELD OF INVENTION
This invention relates to thermoplastic foams and foam articles
having one or more of the following properties: small cells,
uniform cell sizes, pressure sensitive adhesive compositions,
blended immiscible thermoplastic polymer compositions. The
invention further relates to a method for making the foams and a
method for coextruding the foams with other materials.
SUMMARY OF INVENTION
In one aspect, the present invention relates to continuous
processes for producing foams. The processes can be used to produce
foams comprised of amorphous thermoplastic polymers, including
pressure sensitive adhesives, and blends of immiscible polymers.
Another aspect of the invention is a process to coextrude the foams
with other polymeric materials.
In one aspect, the present invention provides a continuous method
for producing a foam material comprising:
(1) mixing at least one amorphous thermoplastic polymeric material
and at least one physical blowing agent in an apparatus having an
exit shaping orifice at a temperature and pressure sufficient to
form a melt solution wherein the blowing agent is uniformly
distributed throughout the polymeric material;
(2) reducing the temperature of the melt solution at the exit of
the apparatus to an exit temperature that is equal to or less than
30.degree. C. above the glass transition temperature of the neat
polymeric material while maintaining the melt solution at a
pressure sufficient to keep the blowing agent in solution; and
(3) passing the solution through the exit shaping orifice and
exposing the solution to atmospheric pressure, thereby causing the
blowing agent to expand resulting in nucleation and cell formation,
which causes the melt solution to foam at or about the time it
exits the shaping orifice.
In another aspect, the invention provides foam-containing articles
that can be designed to exhibit a wide range of properties for a
myriad of applications. The polymeric materials used in making the
articles may comprise amorphous thermoplastic polymers including
pressure sensitive adhesives, and blends of immiscible
thermoplastic polymers. A range of suitable exit temperatures may
be determined based on the polymeric material used to make the
foam.
In another aspect, the invention further provides a way to control
the cell size and cell size distribution of a foam by adjusting,
manipulating, or controlling the blowing agent concentration, the
exit temperature, and/or the exit pressure of the foamable melt
solution.
In another aspect, the invention features articles comprising a
foam having cell sizes of 2 to 200 micrometers, preferably 5 to 50
micrometers. The foam may alternatively, or additionally, have a
cell size distribution with a polydispersity from 1.0 to 2.0,
preferably from 1.0 to 1.5, more preferably from 1.0 to 1.2.
In another aspect, the invention features articles wherein the foam
of the invention comprises at least one layer in a multi-layer
construction.
The invention further features a coextrusion process whereby a foam
is coextruded with at least one other material, which may be a
foamed or unfoamed material.
As used in this invention:
"small-cell foam" means a foam having cell sizes of 2 to 200
micrometers (.mu.m), preferably 5 to 50 .mu.m;
"closed-cell" means a foam material that contains substantially no
connected cell pathways that extend from one outer surface through
the material to another outer surface;
"operating temperature" means the temperature that must be achieved
in the extrusion process, prior to the addition of the physical
blowing agent, to melt all of the polymeric materials in the melt
mix;
"T.sub.g " means the glass transition temperature, i.e., the
temperature at which a polymer changes from a fluid to a solid
state;
"exit temperature" and "exit pressure" mean the temperature and
pressure of the extrudate in the final zone or zones of the
extruder and preferably in the die;
"average" means the arithmetic average, i.e., mean;
"standard deviation" means the "typical" deviation in cell size
from the mean cell size; it is calculated using the following
formula: ##EQU1##
where .sigma. is the standard deviation, x.sub.i is an observed
cell size, x is the arithmetic average cell size, and n is the
total number of cell size observations;
"melt solution" or "melt mixture" or "melt mix" means a
melt-blended mixture of polymeric material(s), any desired
additives, and blowing agent(s) wherein the mixture is sufficiently
fluid to be processed through an extruder;
"neat polymer" means a polymeric material having no additives, and
at standard temperature and pressure;
"nucleation" means a process by which a homogeneous solution of
polymeric material and dissolved molecules of a species that is a
gas under ambient conditions undergoes formations of clusters of
molecules of the species that define "nucleation sites" from which
cells will grow; i.e., it is a change from a homogeneous solution
to a multi-phase mixture in which, throughout the polymeric
material, sites of aggregation of at least several molecules of
physical blowing agent are formed (if immiscible polymeric
materials are used, the physical blowing agent will typically form
single-phase solutions with one or more of the polymer materials,
but the polymers will typically not combine to form a single
phase);
"supercritical fluid" means a substance, which is typically a gas
at ambient temperature and pressure, compressed to a state where it
has the density and solvation characteristics of a liquid, but the
viscosity, permeability, and diffusivity of a gas; a supercritical
fluid is a single phase material that exists above a critical
point, which point is determined by a critical temperature,
T.sub.c, and critical pressure, P.sub.c, which T.sub.c and P.sub.c
depend on the particular gas (for example, the T.sub.c and P.sub.c
for carbon dioxide are approximately 31.degree. C. and 7.4 MPa
(1078 psia), respectively);
"foam density" means the weight of a given volume of foam;
"inversion temperature" means the temperature at which a minimum
foam density is obtained for a given polymeric foam; at
temperatures above and below the inversion temperature, a higher
foam density will typically be obtained;
"density reduction" refers to a way of measuring the void volume of
a foam based on the following formula: ##EQU2##
where .rho..sub.R is the density reduction, .rho..sub.f is the foam
density, and .rho..sub.o is the density of the original
material;
"polydispersity" means the weight average cell diameter divided by
the number average cell diameter for a particular foam sample; it
is a means of measuring the uniformity of cell sizes in the
sample;
"uniform" means that the cell size distribution has a
polydispersity of 1.0 to 2.0;
"spherical" means generally rounded; it may include spherical,
oval, or circular structure;
"fibrillose" means having elongated filament-like or thread-like
structures;
"schistose" means having parallel plate-like ribbons;
"polymer matrix" means the polymeric, or "non-cell," areas of a
foam;
"blend matrix" means the polymeric material having the highest
volume fraction in a melt mixture comprising at least two
immiscible materials;
"immiscible" refers to thermoplastic polymers that will not mix or
remain mixed with each other, although at certain conditions, such
as high temperatures, they might mix, but any such mixture will
typically be thermodynamically unstable and will typically separate
into distinct- phases at lower temperatures;
"miscible" refers to two or more thermoplastic materials that will
form a homogeneous mixture, that is, dissolve in each other;
"anisotropic" means having different properties or degrees of
properties in different directions parallel to a major surface;
and
"straight line tear" means a tear not deviating more than
20.degree., preferably not more than 10.degree., from the direction
in which the tear is initiated.
An advantage of at least one embodiment of the present invention is
the ability to alter, adjust, or control the foam density, average
cell size, and cell size distribution of foams. This allows the
properties of the produced foams to be optimized based on their
intended use.
An advantage of at least one embodiment of the method of the
present invention is that no special nucleation apparatus is
required to nucleate the foam.
An advantage of at least one embodiment of a foam of the present
invention is that uniform cell sizes can provide uniform
characteristics and properties throughout the foam.
An advantage of at least one embodiment of a foam of the present
invention is that small cells, as opposed to larger cells, will not
as easily propagate defects or cracks in the foam structure.
Another advantage of small cell sizes is that thinner foam
substrates can be produced.
An advantage of at least one embodiment of a foam of the present
invention is that a foam comprising a blend of thermoplastic
materials can allow the foam to have beneficial properties of each
material. For example, a foam combining a stiff and strong material
with a flexible and weak material can have strength and
flexibility. In addition, the foam may have anisotropic properties,
which can be used advantageously. For example, a foam that has high
tensile strength in one direction and flexibility in a
perpendicular direction may be used for high strength substrates or
backings.
Another advantage of at least one embodiment of a foam of the
present invention is that the foams may have low and controlled
dielectric constant and high dielectric strength.
An advantage of at least one embodiment of a foam of the present
invention is the ability to be torn by hand in one or both of
directions parallel and perpendicular to the machine direction
wherein the tears are straight-line tears.
An advantage of at least one embodiment of the present invention
comprising pressure sensitive adhesive materials is that the foam
can possess pressure sensitive adhesive properties without
requiring the application of a pressure sensitive adhesive layer to
the foam surface.
An advantage of at least one embodiment of the present invention is
the ability to produce coextruded foams from materials having
disparate temperatures, while maintaining the structure of the
foam. The coextruded foams can provide articles having a variety of
desired properties.
An advantage of at least one embodiment of a foam of the present
invention is that it can provide the insulating properties.
An advantage of at least one embodiment of a foam of the present
invention is that it can provide the conformability and strength of
a cloth without using an expensive woven material.
Other features and advantages of the invention will be apparent
from the following figures, detailed description, and claims.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 illustrates a tandem extruder system that may be used in a
process of the present invention.
FIG. 2 illustrates a single twin screw extruder system that may be
used in a process of the present invention.
FIG. 3 illustrates the direction in which a cross-section of a foam
sample may be viewed in a micrograph. A sample may be viewed
parallel to the machine direction, i.e., the extrusion direction,
or perpendicular to the machine direction.
FIGS. 4a, 4b, and 4c show scanning electron microscopy (SEM)
micrographs (parallel to the machine direction), with a
magnification of 100, of polystyrene foams comprising PS615, a
polystyrene available from Dow Chemical Co., Midland, Mich.,
produced at 117.degree. C,. 99.degree. C., and 96.degree. C.,
respectively, at a CO.sub.2 concentration of about 7.3, 7.6, and
8.7 weight %, respectively, using a tandem extruder process wherein
the first extruder was operated at 25 rpm and the second at 5
rpm.
FIG. 5 is a cell histogram showing average cell size and standard
deviation for polystyrene foams shown in FIGS. 4a-4c comprising
PS615 produced at 117.degree. C., 99.degree. C., and 96.degree. C.
at a CO.sub.2 concentration of about 7.3, 7.6, and 8.7 weight %,
respectively.
FIGS. 6a, 6b, 6c, and 6d show SEM digital image micrographs
(parallel to the machine direction), with a magnification of 100,
of polystyrene foams comprising PS615 produced at temperatures of
101.degree. C., 107.degree. C., 112.degree. C., and 122.degree. C.,
respectively, at a CO.sub.2 concentration of 6.8, 6.4, 6.2, 6.0
weight %, respectively, using a tandem extruder process wherein the
first extruder was operated at 25 rpm and the second at 5 rpm.
FIG. 7 is a cell histogram showing average cell size and standard
deviation for polystyrene foams shown in FIGS. 6a-6d comprising
PS615 produced at temperatures of 101.degree. C., 107.degree. C., 1
12.degree. C., and 122.degree. C. at a CO.sub.2 concentration of
6.8, 6.4, 6.2, 6.0 weight respectively.
FIGS. 8a, 8b, and 8c show SEM digital image micrographs (parallel
to the machine direction), with a magnification of 100, of
polystyrene foams comprising PS615 produced at CO.sub.2
concentrations of 4.4, 5.7, and 6.2 weight % and exit temperatures
of 118.degree. C., 117.degree. C., and 112.degree. C.,
respectively, using a tandem extruder process wherein the first
extruder was operated at 25 rpm and the second at 5 rpm.
FIG. 9 is a cell size histogram showing average cell size and
standard deviation for polystyrene foams shown in FIGS. 8a-8c
comprising PS615 produced at 1 18.degree. C., 117.degree. C., and
112.degree. C. with CO.sub.2 concentrations of 4.4, 5.7, and 6.2
weight %, respectively.
FIGS. 10a, 10b, and 10c show SEM digital image micrographs
(parallel to the machine direction), with a magnification of 1000,
of microcellular foams comprising 100 wt % PS615; 90 wt % PS615 and
10 wt % KD1107 (a styrene-isoprene-styrene di-block/tri-block
copolymer available from Shell Chemical Co., Houston, Tex.); and 80
wt % PS615 and 20 wt % KD1107, respectively.
FIGS. 11a and 11b show transmission electron microscope (TEM)
digital image micrographs (parallel and perpendicular to the
machine direction, respectively) of an immiscible blend of 80:20
PS615:KD1107.
FIG. 12 shows the results of tensile strength (tensile stress) and
elongation (tensile strain) tests in directions parallel and
perpendicular to the machine direction performed on a foam
comprising a blend of 50:50 PS615:KD1107.
FIG. 13 shows the exponential relationship of average cell size to
exit temperature for polystyrene foams comprising PS615 and
produced at 6-7 wt % CO.sub.2, made at various exit temperatures,
using a tandem extruder process wherein the first extruder was
operated at 25 rpm and the second at 5 rpm.
FIG. 14 shows the exponential relationship of average cell size to
blowing agent concentration for polystyrene foams comprising PS615
and various CO.sub.2 concentrations, made at an exit temperature of
112.degree. C.-118.degree. C., using a tandem extruder process
wherein the first extruder was operated at 25 rpm and the second at
5 rpm.
FIG. 15 shows the combined relationships of average cell size to
exit temperature and blowing agent concentration for polystyrene
foams comprising PS615 and made using a tandem extruder process
wherein the first extruder was operated at 25 rpm and the second at
5 rpm.
FIG. 16 shows the relationship between foam density and exit
temperature for a polystyrene foam comprising PS615 and produced
with a 7-9 wt % CO.sub.2 concentration using a tandem extruder
process wherein the first extruder was operated at 25 rpm and the
second at 5 rpm.
DETAILED DESCRIPTION
One aspect of the present invention provides a continuous process
for generating uniform and/or small cell thermoplastic foams using
a tandem or two-stage extrusion system. This process involves
mixing one or more thermoplastic polymeric materials with a
physical blowing agent, e.g., carbon dioxide (CO.sub.2), that is
soluble with at least one of the polymeric materials, to form a
melt solution. The temperature and pressure conditions in the
extrusion system are preferably sufficient to maintain the
polymeric material and blowing agent as a homogeneous solution. The
blowing agent plasticizes, i.e., lowers the T.sub.g of, the
polymeric material. The inventors have found that by adding a
physical blowing agent, the polymeric materials may be processed
and foamed at temperatures considerably lower than otherwise might
be required. The lower temperature can allow the foam to cool and
stabilize (i.e., reach a point of sufficient solidification to
arrest further cell growth and coalescense) soon after it exits the
die, thereby making it easier to arrest cell growth and coalescence
while the cells are smaller and more uniform. Preferably, the
polymeric materials are foamed at or below 30.degree. C. above the
glass transition temperature of the neat polymer thereby producing
desirable properties such as uniform and/or small cell sizes. The
glass transition temperature is that temperature below which free
rotation of polymer molecules cease because of intermolecular
forces; below its glass transition temperature, a polymer has a
very high viscosity and is, for all practical purposes, a solid;
above the glass transition temperature, the polymer is rubbery or
fluid.
As the melt solution exits the extruder through a shaping die, it
is exposed to the much lower atmospheric pressure causing the
blowing agent to expand and come out of the melt solution. This
causes nucleation and cell formation resulting in foaming of the
melt solution. When the melt solution exit temperature is at or
below 30.degree. C. above the T.sub.g of the neat polymeric
material, the increase in T.sub.g of the polymer as the blowing
agent comes out of the solution causes vitrification of the
polymer, which in turn arrests the growth and coalescense of the
foam cells within seconds or, most typically, a fraction of a
second. This preferably resulting in the formation of small and
uniform voids in the polymeric material. When using exit
temperatures further above the T.sub.g of the neat polymer, cooling
of the polymeric material, and, therefore, arresting the growth and
coalescence of cells may take longer. In addition to the increase
in T.sub.g, adiabatic cooling of the foam may occur as the physical
blowing agent expands. The foams are typically and preferably fully
formed at the exit of the die, as soon as the melt solution is
exposed to ambient pressure.
Tandem Extrusion Process 10
FIG. 1 illustrates a tandem extrusion process that can be used to
make the foams of the present invention. To form a melt solution of
polymer and physical blowing agent, polymeric material is initially
fed into a first extruder 14 (typically a single screw extruder)
which softens and conveys the polymeric material. The polymeric
material may be added to extruder 14 in any convenient form,
including pellets, billets, packages, strands, and ropes. Additives
are typically added with the polymer material but may be added
further downstream. The physical blowing agent, typically in a
liquid or supercritical form, is injected near the exit of the
first extruder. Due to the conditions in the extruder, the physical
blowing agent is typically in a supercritical state while in the
extruder.
The polymer materials, additives, and blowing agent are melt-mixed
in the first extruder 14. The purpose of the melt-mixing step is to
prepare a foamable, extrudable composition in which the blowing
agent and other additives, to the extent present, are distributed
homogeneously throughout the molten polymeric material. Specific
operating conditions are selected to achieve such homogeneous
distribution based upon the properties and characteristics of the
particular composition being processed. The operating and exit
pressures in the extruder 14 should be sufficient to prevent the
blowing agent from expanding thereby causing nucleation and cell
formation in the extruder. The operating temperature in the
extruder 14 should be sufficient to melt all of the polymers in the
melt mix.
Next, the melt mix is fed to a second extruder 20 (typically a
single screw extruder). The second extruder 20 is generally
operated at conditions (e.g. screw speed, screw length, pressure,
and temperature) selected to achieve optimum mixing, and to keep
the blowing agent in solution. The extruder 20 typically has a
decreasing temperature profile wherein the temperature of the last
zone or zones will bring the melt solution to the desired exit
temperature.
At the exit end of the second extruder 20, the foamable, extrudable
composition is metered into a die 22 which has a shaping exit
orifice (e.g., an annular, rod, slit die, or shaped profile die).
The temperature within the die 22 is preferably maintained at
substantially the same temperature as the last zone of the
secondary extruder 20; i.e., at the exit temperature. The
relatively high pressure within the extruder 20 and die 22 prevents
nucleation, cell formation, and foaming of the melt mix composition
in the extruder and die. Exit pressure is dependent upon die
orifice size, exit temperature, blowing agent concentration,
polymer flowrate, polymer viscosity, and polymer type. Exit
pressure is typically controlled by adjusting the die orifice size,
but can also be adjusted by altering the exit temperature, blowing
agent concentration, and other variables. Reducing the size of the
die orifice will generally increase exit pressure. As the
composition exits die 22, through the die's shaping orifice, it is
exposed to ambient pressure. The pressure drop causes the blowing
agent to expand, leading to nucleation and cell formation thereby
causing foaming of the melt solution composition at or about the
time it exits the outer opening of the die exit shaping orifice,
i.e., typically within a fraction of a second. The foam 24 is
typically quenched, i.e., brought to a temperature below the
T.sub.g of the polymeric material comprising the foam, within two
centimeters to five of the die exit, more typically and preferably
less than two centimeters, most preferably as the foamable material
exits the die and is exposed to ambient pressure.
The shape of foam 24 is dictated by the shape of the die exit
orifice. A variety of shapes may be produced, including a
continuous sheet, including sheets with patterned profiles, a tube,
a rope, etc. When it is extruded, the melt solution is at the exit
temperature, a relatively low temperature compared to temperatures
at which most extrusion processes are carried out. The exit
temperature is chosen to allow the production of a foam with the
desired cell size and cell size distribution, preferably at or
below 30.degree. C. above the glass transition temperature of the
neat polymeric material. In general, as the blowing agent separates
from the homogeneous solution into a distinct phase, its
plasticizing effect on the polymeric material decreases and the
glass transition temperature of the polymeric material increases.
As the T.sub.g of the polymeric material approaches the T.sub.g of
the neat polymeric material, the blowing agent can not as easily
expand or coalesce because the polymeric material becomes more
viscous. As the foam material cools further, it solidifies in the
general shape of the exit shaping orifice of die 22.
Twin Screw Extrusion Process 40
FIG. 2 illustrates a twin screw extrusion process that can be used
to make the foams of the present invention. A single twin screw
extruder 44 may be used to form a melt solution of polymer and
physical blowing agent. The polymeric material is introduced into
zone 1 of extruder 44. Additives are typically added with the
polymer but may be added further downstream. A blowing agent is
preferably injected at a location downstream from a point at which
the polymer has melted.
The extruder 44 is operated with a generally decreasing temperature
profile. The temperature of the initial zone(s) of the extruder
must be sufficient to melt the polymeric material(s) being used.
The temperature of the final zone or zones of the extruder are set
at temperatures to achieve the desired extrudate exit
temperature.
Using a single twin screw extruder, as compared to using a tandem
process, to produce a homogeneous foamable solution requires mixing
and transitioning from an operating temperature and pressure to an
exit temperature and pressure over a shorter distance. To achieve a
suitable melt mix, approximately the first half of the extruder
screw may have mixing and conveying elements which knead the
polymer and move it through the extruder. The second half of the
screw may have distributive mixing elements to mix the polymer
material and blowing agent into a homogeneous mixture.
As with the tandem process, the operating and exit pressures (and
temperatures) should be sufficient to prevent the blowing agent
from expanding and causing nucleation and cell formation in the
extruder. The operating temperature is preferably sufficient to
melt the polymer materials, while the last zone or zones of the
extruder are preferably at a temperature that will bring the
extrudate to the exit temperature.
At the exit end of the extruder, the foamable, extrudable
composition is metered into a die 50 having a shaping exit orifice.
The foam is generated in the same manner as with the tandem
system.
Coextrusion Process
The inventors found, unexpectedly, that they were able to coextrude
the foams of the invention with materials having substantially
higher processing temperatures from that of the foam, while still
obtaining the desired structures and cell sizes. For example, the
inventors were able to coextrude a foam at 93.degree. C. and
another polymer material at 204.degree. C., a difference of over
110.degree. C. It would be expected that exposing the foam to an
adjacent hot polymer as it is extruded, might cause the foam cells,
especially those in direct contact with the hotter material, to
continue to grow and coalesce beyond their desired sizes or might
cause the foam material to melt or collapse.
The coextrusion process of the present invention may be used to
make a foam material comprising two layers or more. A layered
material or article may be produced by equipping die 22 or 50 with
an appropriate feed block, e.g., a multilayer feedblock, or by
using a multi-vaned or multi-manifold die such as a 3-layer vane
die available from Cloeren, Orange, Tex. Materials or articles
having multiple adjacent foam layers may be made with foam layers
comprising the same or different materials. Foam articles of the
present invention may comprise one or more interior and/or exterior
foam layer(s). In such a case, each extrudable, foamable material
may be processed using one of the above-described extrusion methods
wherein melt mixtures are fed to different inlets on a multi-layer
feedblock, or multi-manifold die, and are brought together prior to
exiting the die. The layers foam in generally the same manner as
described above for the extrusion process. The multi-layer process
can also be used to extrude the foam of this invention with other
types of materials such as unfoamed polymeric materials and any
other type of polymeric material. When a multi-layered article is
produced, it is preferable to form adjacent layers using materials
having similar viscosities and which provide interlayer
adhesion.
If adjacent layers of materials are heated to substantially
different temperatures, a die can be used that will thermally
isolates the different materials until just prior to their exiting
the die, for example the die disclosed in FIG. 4 of U.S. Pat. No.
5,599,602, incorporated by reference. This can diminish or
eliminate negative effects of contacting the different materials
such as melting or collapsing the foam or causing continued cell
expansion coalescense.
Multilayer foam articles can also be prepared by laminating polymer
or nonpolymer layers to a foam core, or by layering extruded foams
as they exit their respective shaping orifices, with the use of
some affixing means such as an adhesive. Other techniques that can
be used include extrusion coating and inclusion coextrusion, which
is described in U.S. Pat. No. 5,429,856, incorporated by
reference.
Process Variables
The present invention shows that blowing agent concentrations, exit
pressure, and exit temperature can have a significant effect on the
properties of the resulting foams including foam density, cell
size, and distribution of cell sizes. A degree of interdependence
also exists between blowing agent concentrations, pressure, and
temperature with regard to processing conditions. The inventors
found, in general, that the lower the exit temperature, the more
uniform, and smaller, the cell sizes of the foamed material. This
is believed to be because the lower the exit temperature, the
quicker the T.sub.g of the foaming material increases to the exit
temperature as the blowing agent leaves the solution, thereby
causing cell growth to be more quickly arrested. The inventors
found that by extruding the material at lower than normal extrusion
temperatures, preferably at or below 30.degree. C. above the
T.sub.g of the neat polymeric material, they were able to produce
foams with small, uniform cell sizes.
The inventors found that varying the exit temperature of the melt
solution while maintaining a relatively constant blowing agent
concentration, (which is achieved by controlling the flow rate of
the blowing agent in relation to the flow rate of the polymeric
material) can cause a change in foam density. At the temperature
ranges used by the inventors, foam density is, in general,
inversely related to the change in exit temperature. This is shown,
for example, by the general trend of the data in Table 1 and FIG.
16 (below about 125.degree. C.).
In addition, as the exit temperature increases above the polymer
T.sub.g, the foam density typically decreases until the inversion
temperature is reached. At that point a further increase in
temperature typically produces an increase in foam density. Foams
having a density of less than about 0.04 grams/cubic centimeter
(g/cc) tend to be mechanically weak. By keeping the exit
temperature of the extrudate near the T.sub.g of the neat polymer,
the inventors were able to achieve desirable densities while still
achieving small and/or uniform cell sizes. Densities of the foams
of the invention may vary widely. In addition, materials providing
high mechanical strength may be used to overcome weaknesses of low
density foams. Foams having densities greater than 0.1 g/cc (gram
per cubic centimeter) can also be made using a process of the
invention. Typically densities of the foams produced in the
examples were in the range of 0.1 g/cc to 0.3 g/cc. These densities
provide foams with better mechanical properties than foams having
lower densities. Some mechanical properties, such as compressive
strength, can also be influenced by cell size. A smaller cell size
will typically provide a higher compressive strength.
The inventors also found that varying the exit temperature at a
relatively constant blowing agent concentration, can have a
noticeable effect on cell size and cell size distribution. FIG. 13
shows average cell size as a function of the exit temperature. At a
relatively constant blowing agent concentration, as the temperature
decreases, the cell size decreases at an exponential rate. FIGS.
4a, 4b, and 4c shows SEMs and FIG. 5 shows cell size histograms for
compositionally identical foams made from melt solutions having
similar blowing agent concentrations of approximately 7 to 9 weight
%, but formed at three different temperatures. At a temperature of
117.degree. C. the average cell size is 56 .mu.m and the
distribution had a polydispersity of 1.60. A decrease in
temperature to 99.degree. C. decreased the average cell size to 39
.mu.m, with a polydispersity of 1.80. A further decrease to
96.degree. C. decreased the average cell size to 11 .mu.m, an
approximately 4-fold decrease from the average cell size obtained
with the 99.degree. C. temperature. Additionally, the distribution
of cell sizes became very uniform at the low temperature with a
polydispersity of 1.04. FIGS. 6a, 6b, 6c, and 6d and 7 show the
same effect for compositionally identical foams over a different
temperature range. At similar blowing agent concentrations of 6 to
7 weight %, decreasing exit temperatures of 122.degree. C.,
112.degree. C., 107.degree. C. and 101.degree. C. produced average
cell sizes of 115, 49, 36, and 28 .mu.m, respectively, and
polydispersities of 1.11, 1.02, 1.03, and 1.02, respectively. This
aspect of the invention provides a means to create not only a small
cell size (less than 200 micrometer), but a uniform cell size
distribution.
The inventors also found that at a relatively constant exit
temperature, a change in blowing agent concentration can affect
cell size and cell size uniformity. FIG. 14 shows average cell size
as a function of blowing agent concentration at similar
temperatures. As blowing agent concentration increases, average
cell sizes decrease at an exponential rate. FIGS. 8a, 8b, and 8c
and 9 show the effect on a foam of a change in blowing agent
concentration at similar temperatures. As the blowing agent
concentration increases from 4.4 to 5.7 to 6.2 weight %, the
average cell sizes decrease from 178 to 84 to 49 .mu.m and have
polydispersities of 1.03, 1.05, and 1.02, respectively. Even though
the polydispersities are approximately equivalent for all three
foams, it can be seen from FIG. 9 that a higher blowing agent
concentration produces a smaller overall range of cell sizes, i.e.,
a smaller standard deviation. However, it should be noted that at
high exit temperatures, as blowing agent concentration increases,
the average cell size may decrease even though polydispersity
increases. This can occur when the range of cell sizes increases,
but the distribution of cell sizes causes the calculated average
cell size to decrease.
The blowing agent concentration in the system is primarily
controlled by the physical blowing agent and polymer flowrates.
However, depending on the operating and exit pressures of the
process, the actual solubility of the physical blowing agent in the
polymer can change. An increase in pressure will increase
solubility, thereby allowing increased CO.sub.2 concentration in
the melt solution. Therefore, it is important to note that by
controlling the pressure in the process, the properties of the foam
material can be manipulated by changing the physical blowing agent
solubility limit of the polymer. Suitable physical blowing agent
concentration typically range from below, to above, the blowing
agent's saturation point in the neat polymer of the material being
used to make the foam, depending on the desired properties of the
resulting foams. Typically, the preferable blowing agent
concentration range is 50 to 110% of what the blowing agent's
saturation level would be in the neat polymer material.
The pressure of the melt solution in the extruder is dependent on
orifice size, amount of blowing agent, polymer flow rate, polymer
viscosity, polymer type, and temperature. Decreasing the size of
the die exit can increase exit and operating pressures. Decreasing
the process and exit temperatures and decreasing blowing agent
concentration can result in higher pressures. The lower the blowing
agent concentration, the more significant the effect of changing
the temperature on the pressure. It should also be noted, that, in
general, at higher pressures, a given polymer can dissolve more
physical blowing agent.
In general, as the melt solution exits the die, it is preferable to
have a large pressure drop over a short distance. Keeping the
solution at a relatively high pressure until it exits the die helps
to form uniform cell sizes. Maintaining a large pressure drop
between the exit pressure and ambient pressure can also contribute
to the quick foaming of a melt solution. The lower limit for
forming a foam with uniform cells will depend on the critical
pressure of the blowing agent being used. In general, the inventors
found that for the polymers used in the examples, the lower exit
pressure limit for forming acceptably uniform cells is
approximately 7 MPa (1000 psi), preferably 10 MPa (1500 psi), more
preferably 14 MPa (2000 psi). FIG. 15 illustrates the combined
effect of exit temperature and blowing agent concentration on cell
sizes for a polystyrene foam. The smallest cell sizes were produced
at low exit temperatures and high blowing agent concentrations.
However, it is believed that at any given temperature and pressure,
there is a blowing agent concentration at and above which
polydispersity will increase because the polymer becomes
supersaturated with blowing agent and a two phase system is
formed.
The mechanical attributes of foam materials are dependent primarily
on their foam density. See Gibson, L. J., Ashby, M. F., Cellular
Solids: Structure & Properties, Cambridge University Press,
Cambridge, United Kingdom, Second Edition, 1997. The inventors have
found a process window that can generate foams with desirable
densities and small cell sizes. FIG. 16 shows foam density as a
function of exit temperature for a PS615 foam produced using a
tandem screw extrusion process. The inventors found that at
temperatures less than 125.degree. C., foam density decreased as
the exit temperature increased. However at 125.degree. C. this
trend will begin to reverse and the density will begin to increase
as the temperature increased. This indicates that the inversion
temperature for PS615 is approximately 125.degree. C. Because
smaller cell sizes and lower polydispersities can be obtained at
lower temperatures, operating in the area of the curve below the
inversion temperature can yield foam materials with superior
mechanical integrity over the foams generated at temperatures above
the inversion temperature. Moderate to high foam densities are
typically preferred for most applications because a higher foam
density generally provides the foam with greater structural
integrity.
The optimum exit temperature, exit pressure, and blowing agent
concentration for a particular foamable material will depend on a
number of factors such as the type and amount of polymer(s) used;
the physical properties of the polymers, including viscosity; the
solubility of the polymer(s) in the blowing agent; the type and
amount of additives used; the thickness of the foam to be produced;
whether the foam will be coextruded with another foam or an
unfoamed material; and the die gap and die orifice design.
Blowing Agents
A physical blowing agent useful in the present invention is any
naturally occurring atmospheric material which is a vapor at the
temperature and pressure at which the foam exits the die. The
physical blowing agent may be introduced, i.e., injected into the
polymeric material as a gas, a supercritical fluid, or liquid,
preferably as a supercritical fluid or liquid, most preferably as a
liquid. The physical blowing agents used will depend on the
properties sought in the resulting foam articles. Other factors
considered in choosing a blowing agent are its toxicity, vapor
pressure profile, ease of handling, and solubility with regard to
the polymeric materials used. Flammable blowing agents such as
pentane, butane and other organic materials, such as
hydrofluorocarbons (HFC) and hydrochlorofluorocarbons (HCFC) may be
used, but non-flammable, non-toxic, non-ozone depleting blowing
agents are preferred because they are easier to use, e.g., fewer
environmental and safety concerns. Suitable physical blowing agents
include, e.g., carbon dioxide, nitrogen, SF.sub.6, nitrous oxide,
perfluorinated fluids, such as C.sub.2 F.sub.6, argon, helium,
noble gases, such as xenon, air (nitrogen and oxygen blend), and
blends of these materials.
Amorphous Polymers
The polymer matrices of foams of the invention may comprise one or
more amorphous polymers. The polymers may be homopolymers or
copolymers, including random and block copolymers.
It may be desirable to use two or more miscible amorphous polymers
having different compositions to achieve unique foam properties. A
wide range of foam physical properties can be obtained by
selectively choosing the amorphous polymer component types and
concentrations. A particular polymer may be selected based upon the
desired properties of a final foam-containing article.
After the amorphous polymer is mixed with the physical blowing
agent to form a melt solution, and prior to exiting the die, the
temperature of the solution is reduced to an exit temperature that
can provide desired cell sizes and cell size distributions
preferably at or below 30.degree. C. above the glass transition
temperature of the neat polymeric material. A mixture of two or
more miscible amorphous polymers will produce a mixture having a
single T.sub.g. This T.sub.g is typically an average, based on the
weight percent of each polymer in the mixture, of the glass
transition temperatures of the component polymers. Suitable
amorphous polymers include, e.g., polystyrenes, polycarbonates,
polyacrylics, polymethacrylics, elastomers, such as styrenic block
copolymers, e.g., styrene-isoprene-styrene (SIS),
styrene-ethylene/butylene-styrene block copolymers (SEBS),
polybutadiene, polyisoprene, polychloroprene, random and block
copolymers of styrene and dienes (e.g., styrene-butadiene rubber
(SBR)), ethylene-propylene-diene monomer rubber, natural rubber,
ethylene propylene rubber, polyethylene-terephthalate (PETG). Other
examples of amorphous polymers include, e.g.,
polystyrene-polyethylene copolymers, polyvinylcyclohexane,
polyacrylonitrile, polyvinyl chloride, thermoplastic polyurethanes,
aromatic epoxies, amorphous polyesters, amorphous polyamides,
acrylonitrile-butadiene-styrene (ABS) copolymers, polyphenylene
oxide alloys, high impact polystyrene, polystyrene copolymers,
polymethylmethacrylate (PMMA), fluorinated elastomers, polydimethyl
siloxane, polyetherimides, amorphous fluoropolymers, amorphous
polyolefins, polyphenylene oxide, polyphenylene oxide-polystyrene
alloys, copolymers containing at least one amorphous component, and
mixtures thereof.
Pressure Sensitive Adhesives Pressure Sensitive Adhesives (PSAs) is
a distinct category of adhesives and a distinct category of
thermoplastics, which in dry (solvent-free) form are aggressively,
and permanently, tacky at room temperature. They firmly adhere to a
variety of dissimilar surfaces upon mere contact without the need
of more than finger or hand pressure. Pressure sensitive adhesives
require no activation by water, solvent, or heat to exert a strong
adhesive holding force toward such materials as paper, cellophane,
glass, wood, and metals. They are sufficiently cohesive and elastic
in nature so that, despite their aggressive tackiness, they can be
handled with the fingers and removed from smooth surfaces without
leaving a residue. PSAs can be quantitatively described using the
"Dahlquist criteria" which maintains that the elastic modulus of
these materials is less than 10.sup.6 dynes/cm.sup.2 at room
temperature. See Pocius, A. V., Adhesion & Adhesives: An
Introduction, Hanser Publishers, New York, N.Y., First Edition,
1997.
The polymer matrices of the foams of the invention may comprise one
or more pressure sensitive adhesive (PSA). It may be desirable to
use two or more PSA polymers having different compositions to
achieve unique foam properties. A wide range of foam physical
properties can be obtained by selectively choosing the PSA
component types and concentrations. A particular polymer may be
selected based upon the desired properties of a final
foam-containing article.
After the PSA polymer is mixed with the blowing agent to form a
melt solution, and prior to exiting the die, the temperature of the
solution may be reduced to an exit temperature that can provide the
desired cell size and cell size distribution, preferably at or
below 100.degree. C. above, more preferably 50.degree. C. above,
and most preferably 30.degree. C. above, the glass transition
temperature of the neat polymeric material. A mixture of two or
more miscible PSA polymers will produce a mixture having a single
T.sub.g. This T.sub.g is typically an average, based on the weight
percent of each polymer in the mixture, of the glass transition
temperatures of the component polymers. Mixtures of immiscible PSA
components will have a distinct T.sub.g for each individual
polymer. When two or more immiscible PSA polymers are used, the
exit temperature is preferably at or below 100.degree. C. above,
more preferably 50.degree. C. above, and most preferably 30.degree.
C. above, the glass transition temperature of the individual
polymer component having the highest T.sub.g.
Suitable polymers can be adhesive polymers (i.e., polymers that are
inherently adhesive), or polymers that are not inherently adhesive
but are capable of forming adhesive compositions when compounded
with tackifiers. Examples of suitable PSA polymers (as long as they
have an appropriate Dahlquist numbers, either inherently or after
being tackified) include acrylics, acrylic copolymers (e.g.,
isooctylacrylate-acrylic acid), amorphous poly-alpha-olefins (e.g.,
polyoctene, polyhexene, and atactic polypropylene), block
copolymer-based adhesives, natural and synthetic rubbers,
styrene-butadiene rubber (SBR), silicone adhesives, ethylene-vinyl
acetate, siloxanes, and epoxy-containing structural adhesive blends
(e.g., epoxy-acrylate and epoxy-polyester blends), acrylic
copolymers such as those described in U.S. Pat. No. 5,804,610,
incorporated by reference, tackified styrenic block copolymers,
polyolefin copolymers, polyureas, polyurethanes, vinyl
ethers,polyisobutylene/butyl rubber,ethylene-propylene-diene rubber
(EPDM), as well as pressure sensitive adhesives disclosed in
copending application Ser. No. 09/091,683, incorporated by
reference, and mixtures of any of the foregoing pressure sensitive
adhesives.
Tackifiers that may be used include, for example, those listed in
the additives section below.
Immiscible Thermoplastic Blends
Immiscible thermoplastic polymer blends may be used for the polymer
matrices of the foams of this invention as long as the polymeric
materials are suitable for melt extrusion processing. It may be
desirable to blend two or more immiscible polymers having different
compositions to achieve unique foam properties. A wide range of
foam physical properties can be obtained by selectively choosing
the blend component types and concentrations. A particular polymer
may be selected based upon the desired properties of a final
foam-containing article. For example blends of the invention may be
made that have tensile strengths greater than 5 MPa.
After the immiscible polymers are mixed with the blowing agent to
form a melt solution, and prior to exiting the die, the temperature
of the solution may be reduced to an exit temperature that can
provide the desired cell size and cell size distribution,
preferably at or below 100.degree. C. above, more preferably
50.degree. C. above, and most preferably 30.degree. C. above, the
glass transition temperature of the neat polymeric material. A
blend of immiscible polymers will have a distinct T.sub.g for each
individual polymer component. When two or more immiscible
thermoplastic polymers, or a copolymer, are used, the exit
temperature is preferably at or below 100.degree. C. above, more
preferably 50.degree. C. above, and most preferably 30.degree. C.
above, the glass transition temperature of the individual polymer
component having the highest T.sub.g.
FIGS. 10a, 10b, and 10c illustrate that changing the concentration
of one component of an immiscible blend of the present invention
from 0 to 10 wt % to 20 wt % can change the cell size and density
while maintaining low polydispersity values. For FIGS. 10a, 10b,
and 10c, respectively, average cell sizes (standard deviation) were
23, 9, 7 .mu.m; polydispersities were 1.04, 1.05, 1.1; and
densities were 0.22, 0.28, and 0.35 g/cc.
Any single component of a blend may comprise greater than zero, but
less than 100 weight % of any one polymer component. Suitable
blends comprise any two or more amorphous thermoplastic polymers as
long as they are immiscible. Pressure sensitive adhesives may also
be used to form immiscible blend foams. Combinations of one or more
immiscible PSA with one or more immiscible non-PSA may be used.
The composition of the melt mixture will depend on the properties
desired in the resulting foam.
Solubility of Blowing Agent with Blend
The present invention provides foams having small cells, uniform
cell sizes, and controllable densities, which foams are made from
materials that typically would not form foams. As previously
discussed, the solubility of a polymer with a blowing agent affects
the temperature at which the polymeric material may be foamed. The
greater the blowing agent solubility of a polymer, the more
plasticized the polymer can become. Increased plasticization allows
for processing and foaming at lower temperatures than those
required or desired for the neat polymer.
In addition, the inventors have found that by blending a
hard-to-foam polymer(s), i.e., those for which the density
reduction achieved by foaming is less than 20%, with easy-to-foam
polymers, i.e., those for which density reductions achieved by
foaming is 20% or greater, they could impart favorable properties
of the hard-to-foam material(s) in the resulting blend foam. The
inventors also found that for the polymers shown in FIGS. 10a-10c,
the density of the foams were typically remained in the range of
about 0.1 g/cc to 0.3 g/cc, but as the concentration of the
hard-to-foam polymer was increased, a point was reached at which
the density of the resulting foam increased. The concentration at
which this occurs will depend on the polymers in the blend.
The polymer blend may comprise greater than zero, but less than
100% of a hard-to-foam polymer. Density reductions of about 0.34 to
90%, as compared to an unfoamed material comprising the same
materials can be achieved depending on the polymer blend
composition and its structure. Generally, as the amount of
hard-to-foam polymer is increased, density decreases.
Blend Morphology
As FIGS. 10a, 10b, 10c, 11a, and 11b illustrate, when immiscible
polymers are used in blends, the materials and articles produced
may comprise foams having a morphology comprising at least two
distinct domains, i.e., two phases: a first domain being
substantially continuous in nature (the blend matrix) and a second
domain being discontinuous or co-continuous and ranging in shape
from spherical to fibrillose or schistose in a direction parallel
to the extrusion, or machine, direction.
FIGS. 10a, 10b, and 10c show SEM micrographs of small cell
immiscible blend foams with different blend compositions of PS615
and KD1107 (100:0; 90:10; 80:20) viewed from a direction parallel
to the machine direction. The cell sizes of the samples are very
uniform with diameters of approximately 7 to 9 microns.
FIGS. 11a and 11b, respectively, are SEM midrograph views of a
PS615 and KD1107 blend foam from directions both parallel to, and
perpendicular to, the machine direction. The dark areas in the
PS615 polymer matrix is a fibrillose to schistose (mainly
schistose) domain comprised of KD1107 polymer, wherein most of the
fibrillose and schistose structures comprising the domain have
extremely small thicknesses of less than 0.05 .mu.m . Preferably
the thicknesses of the of the fibrillose to schistose structures
are less than 5 micrometers, more preferably less than 0.5
micrometers, and most preferably less than 0.05 micrometers.
An oriented spherical or fibrillose to schistose morphology in the
foam blends may be formed as the immiscible blended materials are
extruded through a die. Die extrusion can impose elongational
forces on a foam causing one or both of shear or extensional
deformation in the resulting foam. Orientation of the domains may
occur as the material passes through the die exit shaping orifice
at the exit end of the extruder. Orientation may occur while the
blowing agent is still in solution or as the blowing agent expands
causing nucleation and cell formation, but prior to the foam being
stabilized. Due to the increase in glass transition temperature
that occurs upon foaming, as previously discussed, the cells
typically are fully formed in their oriented state within seconds
or a fraction of a second, typically a fraction of a second, after
the material exits the die. The amount and type of polymers used
can change the morphology of the resulting foam. Morphology can
also be influenced by orientation of the foam cells, i.e.,
voids.
The properties of the resulting foam may be isotropic, which
typically occurs when the discontinuous phase is spherical; or may
be anisotropic, which typically occurs when the discontinuous phase
is oriented, e.g., fibrillose to schistose. It is possible to
produce blended foams with anisotropic tensile and elongation
properties. Anisotropy of tensile and elongation properties is an
unusual characteristic wherein the force necessary to break or
stretch the foam material or article varies when measured along
different axes. That is, the foam material or article displays
different tensile and elongation characteristics when pulled in
different directions. For example, the inventors were able to
produce blended foams that had tensile strengths in a direction
parallel to the machine direction that were at least three times
greater than tensile strengths in a direction perpendicular to the
machine direction. This is illustrated by the data represented in
FIG. 12, which shows high tensile and low elongation properties
perpendicular to the machine direction and low tensile and high
elongation properties parallel to the machine direction. The
tensile strength of a foam may be influenced by the type of
material(s) used, their concentrations, the length to diameter
ratio of the discontinuous domains, and the break elongation of the
components.
Coextrudable Materials
The foams of the present invention may be coextruded with other
materials including foaming and nonfoaming polymeric materials. The
foam of the invention may be any one or more of a multilayered
structure, which may include one or more layers of material
coextruded with the foam. The coextrudable material may be any
polymeric material that can be used in a hot melt process, i.e.,
any thermoplastic material such as those described herein,
including pressure sensitive adhesives.
Additives
The foamable melt mix may also include additives. Examples of
suitable additives include tackifiers (e.g., rosin esters,
terpenes, phenols, and aliphatic, aromatic, or mixtures of
aliphatic and aromatic synthetic hydrocarbon resins), plasticizers
(other than physical blowing agents), nucleating agents (e.g.,
talc, silicon, or TiO.sub.2), pigments, dyes, reinforcing agents,
solid fillers, hydrophobic or hydrophilic silica, calcium
carbonate, toughening agents, flame retardants, antioxidants,
finely ground polymeric particles (e.g., polyester, nylon, or
polypropylene), expandable microspheres, glass beads, stabilizers
(e.g., UV stabilizers), and combinations thereof.
Chemical blowing agents may also be used in the melt mixture.
Suitable chemical blowing agents include a sodium bicarbonate and
citric acid blend, dinitrosopentamethylenetetramine,
p-toluenesulfonyl hydrazide, 4-4'-oxybis(benzenesulfonyl hydrazide,
azodicarbonamide (1,1'-azobisformamide), p-toluenesulfonyl
semicarbazide, 5-phenyltetrazole, 5-phenyltetrazole analogues,
diisopropylhydrazodicarboxylate,
5-phenyl-3,6-dihydro-1,3,4-oxadiazin-2-one, and sodium
borohydride.
Semi-crystalline materials may also be added to the melt mixture in
amounts of less than 50 volume % of the mixture. Suitable
semi-crystalline materials include polyethylene, polypropylene,
polymethylpentene, polyisobutylene, polyolefin copolymers, Nylon 6,
Nylon 66, polyester, polyester copolymers, fluoropolymers, poly
vinyl acetate, poly vinyl alcohol, poly ethylene oxide,
functionalized polyolefins, ethylene vinyl acetate copolymers,
metal neutralized polyolefin ionomers available under the trade
designation SURLYN from E.I. DuPont de Nemours, Wilmington, Del.,
polyvinylidene fluoride, polytetrafluoroethylene, polyformaldehyde,
polyvinyl butyral, and copolymers having at least one
semi-crystalline compound.
The additives may be added in amounts sufficient to obtain the
desired properties for the foam being produced. The desired
properties are largely dictated by the intended application of the
foam or foam article.
Articles
The invention features materials and articles that comprise a
polymer foam. The foam may be provided in a variety of shapes,
including a rod, a tube, a sheet, including a sheet having a
patterned profile, etc., depending on the die shape. In some
embodiments, e.g., when the foam is provided in the form of a sheet
or a tube, the foam will have two major surfaces.
Examination of the foams by electron microscopy reveals that the
preferable foam structure is characterized by cell sizes of 2 to
200 .mu.m, more preferably cell sizes of 5 to 50 .mu.m. The
preferable foam structures may also be characterized by cell size
distributions having a polydispersity from 1.0 to 2.0, preferably
from 1.0 to 1.5, and most preferably from 1.0 to 1.2.
Foams of the invention that were tested for dielectric properties
were shown to have high dielectric strengths and low dielectric
constants. The inventors were able to manipulate the dielectric
constant by changing foam density. A higher density generally
provided a higher dielectric constant.
Layered articles may be made using the coextrusion process of the
invention. The layered articles may have a myriad of different
properties depending on the materials used and the cell sizes and
cell size distributions of the foams in the articles.
The foams comprised of pressure sensitive adhesives can provide
adhesive foam articles that do not required the separate
application of an adhesive layer.
The foams comprised of blends of immiscible polymers can provide
articles with many uses. The anisotropic properties of some of the
foam blends can be used advantageously in applications where
different properties or degrees of properties are desired in
different directions of the article. For example, it can be useful
to have an article that may be stretched or broken in one
direction, but not another. The ability of some of the blends to be
hand-torn in a straight line (i.e., the tear not deviating more
than 20.degree., preferably not more than 10.degree., from the
direction in which the tear is initiated) both parallel and
perpendicular to the machine direction could also be used
advantageously, such as for a tape substrate.
After the foam material or article has been formed, it may be
subjected to further processing. For example, the foam may be
subjected to ultraviolet or actinic radiation, an electron beam
source, or a thermal, e.g., infra red, source to crosslink the
polymeric materials in the foam. The foam material may also be
subjected to post-production orientation, lamination, replication,
converting, or thermoforming.
EXAMPLES
Test Methods
Foam Density (ASTM D792-86)
Foam samples were cut into 12.5 mm.times.25.4 mm specimens and
weighed on a high precision balance available as Model AG245 from
Mettler-Toledo, Greifensee, Switzerland. The volume of each sample
was obtained by measuring the mass of water displaced at room
temperature (25.degree. C.). Assuming the density of water at
25.degree. C. to be 1 g/cm.sup.3, the volume of each sample was
calculated using Archimede's principle. The density of the foam was
obtained by the quotient of the mass and volume. Accuracy of this
measurement is .+-.0.005 g/cm.sup.3.
Foam Cell Size
Scanning electron microscopy was performed on all the foam samples
using a scanning electron microscope available as model JSM-35C
from JEOL, Peabody, Massachusetts, operated at 5 and 10 kV. The
samples were prepared by freezing in liquid nitrogen for 2-5
minutes and fracturing. A thin palladium-gold coating was
evaporated on the samples to develop a conductive surface. The
diameters of the foam cells were measured using the digital SEM
micrographs and UTHSCSA Image Tool for Windows Software (Version
1.28, University of Texas, San Antonio, Tex.). The diameters of
over 100 cells were measured and recorded. The average cell
diameter (x) and standard deviation (.sigma.) of the cell diameter
was calculated using the Image Tool Software.
Foam Uniformity--Polydispersity
The number (N.sub.i) of foam cells with diameter (x.sub.i) for each
foam sample was measured using digital SEM images and the UTHSCSA
Image Tool software, as described above.
The number average diameter, x.sub.n, of the sample was calculated
using the following relationship, ##EQU3##
where N.sub.i is the number of cells with diameter x.sub.i. The
weight average diameter, x.sub.w, of the foam sample can also be
calculated using the relationship below, ##EQU4##
where N.sub.i is the number of cells with diameter x.sub.i. The
polydispersity index, P, is the ratio of the weight average cell
diameter and the number average cell diameter as shown below,
##EQU5##
For polydisperse systems the amount by which P deviates from unity
is a measure of the variation of the cell diameters. In the event
that all the cell diameters have the same diameter, P would be one.
Such a sample is said to be monodisperse. See Hiemenz, P. C.,
Polymer Chemistry, Marcel Dekker Inc., New York, N.Y., 1984, pp.
34-55.
Tensile Strength and Elongation
The foam tensile and elongation properties, as defined in ASTM
D638-95, were measured at room temperature using a testing device
available as Model 55R1122, from Instron, Canton, Mass. The samples
were first conditioned at 21.degree. C. and 50% humidity for 5
days. The samples were then cut into 130 mm.times.12.5 mm
specimens. The thickness of each specimen was measured using a
digital linear gauge available as Model EG-233 from Ono Sokki,
Tokyo, Japan, and recorded. The samples were tested using gauge
lengths of 51 mm at a rate of 254 mm/min until failure. The
strength (.sigma.) was measured as a function of elongation
(.epsilon.). The maximum values of .sigma. and .epsilon. are
reported as .sigma..sub.max and .epsilon..sub.max,
respectively.
Dielectric Constant and Loss Tangent
The dielectric constant and loss tangent (tan .delta.) were
obtained by cutting a 12.5 mm.times.12.5 mm foam specimen and
placing the sample in a 4.8 mm (5/16") diameter dielectric test
fixture available as HP16453A from Hewlett-Packard, Palo Alto,
Calif. The capacitance based measurements were obtained using a
HP4291A impedance materializer at frequencies of 1-1800 Megahertz
(MHz).
Dielectric Strength
Dielectric strength measurements were obtained using a Phenix
Technologies (Accident, Md.) Model 6100-510149 dielectric tester
with the ends of 6.35 mm diameter brass rods as the capacitor
plates. Voltage ramp rates were 500 volts/second (v/s). All the
measurements were run in a fluorinated fluid available as FLURINERT
FC-40 from 3M Company, St. Paul, Minn.
Transmission Electron Microscopy
Transmission Electron Microscopy (TEM) samples were prepared by
cutting large foam samples while immersed in liquid nitrogen. These
samples were further trimmed in the shape of a triangle with one
very long and thin point. The samples were then embedded into an
electrical resin available as SCOTCHCAST electrical resin #5 from
3M Company, St. Paul, Minn., and thermally cured for 24 hours.
Sections with varying thickness (85-95 nanometers) were
ultramicrotomed using a Leica Reichert Ultracut T with FCS
(Wetzlar, Germany) at -40.degree. C. using a diamond knife. The
microtomed samples were placed on a 200 mesh copper grid with
carbon stabilized formvar (a polyvinyl aldehyde) substrates,
brought to room temperature and exposed to osmium tetraoxide
(OsO.sub.4) for 2 hours. TEM micrographs were collected using a
JEOL 200CX microscope at 200 kV.
Materials Used Material Description Tg .degree. C. Dow PS615 A
polystyrene available from Dow 105 Chemical Co., Midland, MI
CO.sub.2 Carbon dioxide gas, at 830 psig, avail- N/A able from
Oxygen Services Company, St. Paul, MN EASTAR A copolyester,
available from Eastman 81 PETG 6763 Chemical Co., Kingsport, TN
KRATON a styrene-isoprene-styrene di-block/tri- iso = -73 D1107
block copolymer availabie from Shell sty = 105 Chemical Co.,
Houston, TX TENITE 1550P a low density polyethylene available -125
from Eastman Chemical Co., Kingsport, TN VECTOR 4211 a
styrene-isoprene-styrene di-block/tri- iso = -73 blockcopolymer
available from DexCo., sty = 105 Houston, TX HL-2647 A rubber
pressure-sensitive adhesive iso = -12.5 available from H.B. Fuller
Chemical sty = 105 Co., St. Paul, MN Dow PE 6806 a linear low
density polyethylene avail- -125 able from Dow Chemical Co.,
Midland, MI IOA/AA Isooctyl acrylate - acrylic acid copoly- IOA =
-40 mer made using pouch polymerization AA = 100 process described
in U.S. Pat. No. 5,804,610
Tandem Single Screw Extrusion Process 10
Polymer pellets were fed into a gravimetric batch blender 12
available as Model ACW-T from ConAir-Franklin, Media, Pa. Any
additional solid components were fed into separate chambers in the
gravimetric batch blender. The desired ratio of components was
programmed into the blender controller. The blender fed the
components to a first single screw extruder 14, an NRM single screw
extruder available from Davis-Standard, Pawcatuck, Conn. The
extruder 14 had 6 zones, a 64 mm (2.5") diameter, a length to
diameter ratio of 36:1 and a two-stage screw having a compression
ratio of 3:1, available as Model PS-31 from Plastic Engineering
Associates, Inc., Boca Raton, Fla. The extruder 14 was typically
operated at 25 rpm with an increasing temperature profile from zone
1 to zone 6, with temperatures set for each zone of extruder 14,
which created increasing operating pressures from zone 1 to zone
6.
A physical blowing agent (PBA), typically CO.sub.2, was injected
into extruder 14 between zones 5 and 6, between two blister rings
on the screw, by a laboratory injection system 16 available as
Model 567 from Sencorp Systems Inc., Hyannis, Mass. The physical
blowing agent injection rates were controlled to concentrations
between 1 and 20 weight % of the total polymer flowrate. The
polymer-PBA mixture was mixed to form a melt solution then conveyed
through a 25 mm diameter transfer pipe 18 to a second single screw
extruder 20, a 89 mm diameter (3.5") NRM Davis-Standard single
screw extruder. The extruder 20 had 6 zones, a length to diameter
ratio of 30:1, and a screw having distributive mixing elements
along substantially the entire length of the screw, available as
Model SFS-43 from Plastic Engineering Associates, Inc.
Extruder 20 was typically operated at 5 rpm with a decreasing
temperature profile from zone 1 to zone 6. Temperatures were set
for each of zones 1 to 6 with zone 6 having the desired exit
temperature. The pressure of the blowing agent entering extruder 14
from injection system 16 was adjusted to maintain the blowing agent
concentration as the downstream temperatures in extruder 20 were
changed. The pressure in extruder 20 was maintained at a level that
would prevent nucleation until the melt solution exited die 22. Die
22 was attached to the exit end of extruder 20. The shape of the
exit shaping orifice of the die used depended on the desired shape
of the extruded foam. The melt mixture was fed to die 22 at a
constant flowrate, which created a pressure at the entrance of die
22.
As the melt solution exited the die and was exposed to atmospheric
pressure of approximately 0.104 MPa (15 psi) the PBA expanded and
nucleation and cell growth occurred, forming a foamed material 24
from the melt solution. The foamed material 24 optionally may be
passed through a nip roll 26 and onto a winder 28.
Single Twin Screw Extruder Process 40
Polymer pellets were fed through a gravimetric solids feeder 42
available as model T-35 from Ktron America, Pitman, N.J. into a 40
mm diameter co-rotating twin screw extruder 44 available as model
ZE-40 from Berstorff, Florence, Ky. The twin screw extruder 44 had
10 zones, and a length to diameter ratio of 40:1. Extruder 44 was
typically operated at 55 rpm. The screw in the extruder comprised
forward kneading segments in Zones 3 and 6, a reverse kneading
segment in Zone 4, pin mixers in Zones 7, 8, 9, and 10, and a
blister ring between Zones 4 and 5. The remaining Zones comprised
conveying elements. A physical blowing agent, typically CO.sub.2,
was metered into Zone 5 using laboratory injection system 46,
available as model 567 from Sencorp Systems, Inc., Hyannis, Mass.
The screw design was configured to quickly melt and knead the
polymer in the initial zones of the extruder to create a polymer
seal prior to injection of the physical blowing agent. The design
of the screw downstream of the blowing agent injection was to
facilitate mixing and distribution of the blowing agent into the
polymer. The extruder was operated with a generally decreasing
temperature profile. After the melt mixture passed through the
extruder, it entered a 10.3 cm.sup.3 /revolution polymer melt pump
48, available from Normag, Santa Clarita, Calif., which controlled
the flow rate of the mixture to an exit shaping die 50. The die was
typically a 4.8 mm capillary die. As the melt mixture exited the
die, the physical blowing agent expanded causing nucleation and
cell formation whereby the melt solution foamed, forming circular
foam rods 52. The foam rods were then broken into pieces and
collected.
Examples 1-4
Examples 1-4 illustrate the effect of varying exit temperature on
foam properties.
In Example 1, the tandem single screw extrusion process was used to
make a foam comprising an amorphous thermoplastic polymeric
material (DOW PS615). The temperature profile of extruder 14 for
zones 1 to 6, respectively, was 192.degree. C., 204.degree. C.,
232.degree. C., 232.degree. C., 227.degree. C., and 227 .degree. C.
The physical blowing agent (PBA), CO.sub.2, was injected at a
pressure of approximately 17 MPa (2448 psi), which was
approximately 3 MPa (500 psi) greater than the operating pressure
in extruder 14. The temperature profile of extruder 20 for zones 1
to 6, respectively, was 216.degree. C., 216.degree. C., 176.degree.
C., 122.degree. C., 121.degree. C. The temperature of die 22 was
121.degree. C. The exit temperature of the melt solution was
117.degree. C. The exit temperature of the melt solution was
measured by a thermocouple extending into the melt stream in the
last zone of the extruder. The temperature of the extrudate at the
exit of the die was determined by infrared analysis. The extrudate
temperature at the die exit was typically within approximately
1.degree. C. of the exit temperature. Die 22 was a 25 mm wide slit
die having a gap of 0.61 mm. The exit pressure of the extrudate was
measured by a transducer in the last zone of the extruder. Other
operating conditions and measured properties are shown in Table
1.
Examples 2-4 were made by the method described in Example 1 except
the temperatures in zones 4 to 6 of extruder 20 and die 22 were
reduced to 110.degree. C., 104.degree. C., 99.degree. C., and
99.degree. C., respectively, and the exit temperatures and
pressures differed as shown in Table 1. Other operating conditions
are also shown in Table 1.
Foam density, cell size, and polydispersity for examples 1-4 are
shown in Table 1.
TABLE 1 Polymer CO.sub.2 Flow- Flow- Concen- Cell size Com- rate
rate tration T.sub.exit T.sub.exit -Tg P.sub.exit Density Ave. Std.
Dev. Ex. position (kg/hr) (kg/hr) (wt %) (.degree. C.) (.degree.
C.) (MPa) (g/cc) (.mu.m) (.mu.m) P 1 100% 20.2 1.54 7.3 117 +12
10.8 0.04 56 45 1.61 PS615 2 100% 19.9 1.59 7.4 106 +0.6 12.3 0.07
55 43 1.5 PS615 3 100% 19.4 1.59 7.6 99 -6 12.8 0.10 39 35 1.8
PS615 4 100% 16.8 1.59 8.7 96 -9 14.8 0.16 11 2 1.04 PS615 5 100%
19.5 1.25 6.0 122 +17 10.4 0.04 115 39 1.11 PS615 6 100% 19.0 1.25
6.2 112 +7 12.8 0.04 49 7 1.02 PS615 7 100% 18.1 1.25 6.4 107 +2
14.9 0.06 36 7 1.03 PS615 8 100% 17.0 1.25 6.8 101 -4 16.7 0.11 28
4 1.02 PS615 9 100% 19.7 1.82 8.4 80 -25 13.8 0.22 19 5 1.07
PS615
As seen in Table 1, by comparing examples 1-4 and examples 5-8, as
the exit temperature is reduced while physical blowing agent
concentrations stayed relatively constant, foam density increased
and average cell diameter decreased. Example 9 illustrates that the
process will work at an exit temperature that is 25 degrees below
the glass transition temperature of the neat polymer.
Examples 10-14
Examples 10-14 illustrate the effect of varying blowing agent
concentration on foam properties.
Examples 10-13 were made in a manner similar to Example 1 except
the temperature in extruder 20 was held constant through zones 4 to
6 at 121.degree. C., 121.degree. C., 116.degree. C. and 127.degree.
C., respectively, while blowing agent concentrations were changed.
Example 14 was made under the same conditions as Example 1. Other
operating conditions are shown in Table 2. The foams in examples
10-14 were produced in the shape of a solid rope using a 4.8 mm
capillary die.
Foam density, cell size, and polydispersity for examples 10-14 are
shown in Table 2.
TABLE 2 Polymer CO.sub.2 Cell size Com- Flowrate Flowrate Conc.
T.sub.exit T.sub.exit -Tg P.sub.exit Density Ave. Std. Dev. Ex.
position (kg/hr) (kg/hr) (wt %) (.degree. C.) (.degree. C.) (MPa)
(g/cc) (.mu.m) (.mu.m) P 10 100% 17.1 0.79 4.4 118 +13 16.6 0.04
178 30 1.03 PS615 11 100% 17.0 1.02 5.7 117 +12 14.7 0.056 84 19
1.05 PS615 12 100% 19.0 1.25 6.2 112 +7 12.8 0.041 49 7 1.02 PS615
13 100% 19.4 1.36 6.6 120 +15 9.9 0.066 84 39 1.21 PS615 14 100%
20.2 1.59 7.3 117 +12 10.8 0.073 56 45 1.61 PS615
As seen in Table 2, with a relatively constant exit temperature, an
increase in blowing agent concentration generally results in a
decrease in average cell diameter and low polydispersities except
an increase in polydispersity is shown at higher blowing agent
concentrations.
Example 15-25
Examples 15-25 illustrate the properties of foams made from a blend
of two immiscible polymeric materials.
Each example was made in a manner similar to Example 1 except 1)
under different operating conditions as shown in Table 3, 2) using
a 32 mm diameter single spider annular die available as Model 567
from Sencorp Systems, Inc. and 3) with different temperature
profiles. Because PS615 had the highest T.sub.g of the polymers
used, it was used to calculate the appropriate exit temperature
range.
For each of examples 15, 16, 17, 18, 19, 20, and 21, the
temperature profile of extruder 14 for zones 1 to 6 was 193.degree.
C., 204.degree. C., 232.degree. C., 233.degree. C., 227.degree. C.,
respectively; the temperature profile of extruder 20 for zones 1 to
6 was 216.degree. C., 216.degree. C., 193.degree. C., 99.degree.
C., 99.degree. C. and 99.degree. C., respectively.
For example 22 the temperature profile of extruder 14 for zones 1
to 6 was 193.degree. C., 205.degree. C., 232.degree. C.,
232.degree. C., 227.degree. C. and 227.degree. C, respectively; the
temperature profile of extruder 20 for zones 1 to 6 was 216.degree.
C., 216.degree. C., 193.degree. C., 116.degree. C., 116.degree. C.,
respectively.
For examples 23 and 24, the temperature profile of extruder 14 for
zones 1 to 6 was 193.degree. C., 204.degree. C., 232.degree. C.,
232.degree. C., 227.degree. C. and 227.degree. C., respectively;
the temperature profile of extruder 20 for zones 1 to 6 was
216.degree. C., 216.degree. C., 193.degree. C., 116.degree. C.,
116.degree. C., respectively.
For example 25, the temperature profile of extruder 14 for zones 1
to 6 was 193.degree. C., 204.degree. C., 232.degree. C.,
232.degree. C., 227.degree. C. and 227.degree. C., respectively;
the temperature profile of extruder 20 for zones 1 to 6 was
216.degree. C., 216.degree. C., 193.degree. C., 104.degree. C.,
104.degree. C., respectively.
Foam density, cell size, and polydispersity for examples 15-25 are
shown in Table 3. Dielectric constant, loss tangent (tan .delta.)
(reported in milli units(10.sup.-3)), tensile strength (measured in
volts/mil, i.e., volts/25 micrometers), and anisotropic properties
for examples 15-20 are shown in Table 3A.
TABLE 3 Polymer CO.sub.2 Cell size Flowrate Flowrate Conc.
T.sub.exit T.sub.exit -Tg P.sub.exit Density Average Std. Dev. Ex.
Composition (kg/hr) (kg/hr) (wt %) (.degree. C.) (.degree. C.)
(MPa) (g/cc) (.mu.m) (.mu.m) P 15 100% PS615 18.7 1.25 6.3 96 -9
15.5 0.22 23 5 1.04 16 90% PS615/ 18.7 1.25 6.3 96 -9 18.1 0.27 10
3 1.07 10% Kraton D 1107 17 80% PS615/ 18.7 1.25 6.3 96 -9 19.9
0.35 7 2 1.1 20% Kraton D 1107 18 70% PS615/ 14.8 1.25 7.8 93 -12
19.9 0.29 15 3 1.04 30% Kraton D 1107 19 60% PS615/ 14.8 1.25 7.8
94 -11 19.9 0.30 34 7 1.04 40% Kraton D 1107 20 50% PS615 15.1 1.25
7.6 94 -11 17.9 0.42 9 3 1.09 50% Kraton D 1107 21 40% PS615/ 15.3
1.25 7.6 94 -11 17.7 0.35 16 5 1.07 60% Kraton D 1107 22 10% PS615/
17.4 1.25 6.7 101 -5 13.8 0.72 39 13 1.1 90% Kraton D 1107 23 90%
PS615/ 14.8 1.25 7.8 111 +6 17.6 0.15 21 16 1.6 10% Tenite 1550P 24
90% PS615/ 18.1 1.25 6.5 111 +6 15.4 0.05 36 11 1.08 10% PETG 6763
25 70% PS615/ 13.0 1.25 8.9 101 -4 22.3 0.23 36 8 1.05 30% Vector
4211
TABLE 3A dielectric dielectric tensile strength Tensile strength MD
elong. ratio constant tan .delta. strength parallel to machine
ratio parallel MD/ elong. perpendicular Ex. Comp. (@ 900 MHZ) (mu)
(v/mil) direction (MPa) perpendicular MD % MD/parallel MD 15 100%
PS615 1.26 0.36 499 8.7 6.7 5.0 0.5 16 90% PS615/ 1.38 0.15 321
12.0 6.2 13.4 0.31 10% Kraton D 1107 17 80% PS615/ 1.50 0.14 472
14.8 13.4 13.6 0.40 20% Kraton D 1107 18 70% PS615/ 1.93 0.29 546
17.8 11.9 20.0 1.25 30% Kraton D 1107 19 50% PS615/ 2.45 0.17 608
11.0 1.2 20.0 6.2 50% Kraton D 1107 20 40% PS615/ 3.05 1.74 574
11.4 1.9 23.3 6.4 60% Kraton D 1107
As seen in Table 3, immiscible thermoplastic blends can be used to
make foams having small and uniform cell sizes. Table 3A shows that
the foams may also exhibit useful properties such as a dielectric
constant between 1.25 and 3, high dielectric strength, high tensile
strength, and anisotropic strength and elongation properties.
Example 26-30
Examples 26-30 illustrate the properties of foams made with
pressure-sensitive adhesive polymeric materials.
The pressure sensitive adhesives used in examples 26 to 28 were a
pressure-sensitive adhesive composition prepared by mixing 85 parts
of IOA (isooctyl acrylate), 15 parts of AA (acrylic acid), 0.15
part 2,2 dimethoxy-2-phenylacetophenone (IRGACURE 651 available
from Ciba Geigy) and 0.03 parts of IOTG (isooctyl thioglycolate).
The composition was placed into packages measuring approximately 10
cm by 5 cm by 0.5 cm thick packages as described in Assignee's
co-pending patent application Ser. No. 08/919,756. The packaging
film was a 0.0635 thick ethylene vinylacetate copolymer (VA-24 Film
available from CT Film of Dallas, Tex.) The packages were immersed
in a water bath and at the same time exposed to ultraviolet
radiation at an intensity of 3.5 milliwatts per square centimeter
and a total energy of 1627 millijoules per square centimeter as
measured in NIST units to form a packaged
pressure-sensitive-adhesive. The resulting adhesive had an IV
(intrinsic viscosity of about 1.1 deciliters/gram, Mw of
5.6.times.10.sup.5 g/mol, and Mn of 1.4.times.10.sup.5
g/mol.reacted in an ethylene vinyl acetate pouch according to the
method described in U.S. Pat. No. 5,804,610.
The pressure sensitive adhesives used in example 29 was made in the
same manner as those for examples 26-28, but the IOA/AA ratio was
80:20.
Examples 26 to 28 were made as in Example 1 except under different
operating conditions as shown in Table 4. For examples 26 and 27,
the temperature profile of extruder 14 for zones 1 to 6 was
121.1.degree. C., 160.degree. C., 176.7.degree. C., 176.7.degree.
C. and 176.7.degree. C., respectively; the temperature profile of
extruder 20 for zones 1 to 6 was 176.7.degree. C., 176.7.degree.
C., 160.degree. C., 148.9.degree. C, 148.9.degree. C. and
148.9.degree. C., respectively.
In addition, the samples were made using a 32 mm diameter single
spider annular die available as Model 567 from Sencorp Systems,
Inc., Hyannis, Minn., which produced the foam in the form of a
hollow tube. Within 30 minutes of being extruded, the pressure
sensitive adhesive foams were exposed to an electron-beam source to
crosslink the polymers of the foam. The foams were placed on a
carrier web and passed at 7.6 m/min through an accelerated electron
source available as ESI ELECTROCURTAIN from Energy Sciences, Inc.,
Woburn, Mass. A nitrogen atmosphere was maintained in the
accelerator. The electron beam accelerator was operated at 300 keV
and dosages for examples 26, 27, and 28 were 60, 120, and 180
kiloGrays, respectively.
Examples 29 and 30 were made in the same manner as examples 26-28,
but were not exposed to an electron beam source. In addition,
example 30 used a different PSA (40% PS615/60% KD1107).
Foam density, cell size, and polydispersity, for examples 26-30 are
shown in Table 4.
TABLE 4 Polymer CO.sub.2 Cell size Flowrate Flowrate Conc.
T.sub.exit T.sub.exit -Tg P.sub.exit Dose Density Ave. Std. Dev.
Ex. Composition (kg/hr) (kg/hr) (wt %) (.degree. C.) (.degree. C.)
(MPa) (kGy) (g/cc) (.mu.m) (.mu.m) P 26 85% IOA/ 10.8 0.68 5.9 141
+41 9.2 60 0.83 28 8 1.09 15% AA 27 85% IOA/ 10.8 0.68 5.9 141 +41
9.2 120 0.74 24 12 1.22 15% AA 28 85% IOA/ 10.8 0.68 5.9 141 +41
9.2 189 0.72 19 9 1.2 15% AA 29 80% IOA/ 8.9 1.59 15.1 142 +41 7.5
0 0.84 43 33 1.57 20% AA 30 40% PS615/ 15.3 1.25 7.0 94 -11 17.6 0
0.35 16 5 1.07 60% Kraton D1107
As seen in Table 4 pressure sensitive adhesive materials can be
used to make foams having small and uniform cell sizes.
Examples 31-33
Examples 31-33 illustrate the method using a single twin screw
extruder. In Examples 31-33, single twin screw extruder process 40,
previously described, was used to make the foamed articles.
For example 31, extruder 44 had a temperature profile for zones 1
to 10 of 215.degree. C., 205.degree. C., 202.degree. C.,
172.degree. C., 135.degree. C., 121.degree. C., 113.degree. C.,
84.degree. C., 78.degree. C., and 85.degree. C., respectively. The
exit temperature of the melt solution was 108.degree. C. For
example 32, extruder 44 had a temperature profile for Zones 1 to 10
of 214.degree. C., 204.degree. C., 202.degree. C., 174.degree. C.,
135.degree. C., 125.degree. C., 112.degree. C., 93.degree. C.,
88.degree. C., and 102.degree. C., respectively. The exit
temperature of the melt solution was 125.degree. C. For example 33,
extruder 44 had a temperature profile for Zones 1 to 10 of
215.degree. C., 205.degree. C., 201.degree. C., 175.degree. C.,
122.degree. C., 130.degree. C., 131.degree. C., 110.degree. C.,
111.degree. C., and 120.degree. C., respectively. The exit
temperature of the melt solution was 139.degree. C. In these
examples, the melt solution was extruded through a 4.8 mm diameter
capillary die.
Foam density, cell size, polydispersity for examples 31-33 are
shown in Table 5.
TABLE 5 Polymer CO.sub.2 Cell size Flowrate Flowrate Conc.
T.sub.exit T.sub.exit -Tg P.sub.exit Density Average Std. Dev. Ex.
Composition (kg/hr) (kg/hr) (wt %) (.degree. C.) (.degree. C.)
(MPa) (g/cc) (.mu.m) (.mu.m) P 31 100% PS615 9.1 1.36 13.0 108 +3
9.3 0.11 50 28 1.31 32 100% PS615 9.1 1.36 13.0 125 +20 7.4 0.05 27
8 1.08 33 100% PS615 9.1 1.36 13.0 139 +34 6.1 0.03 165 89 1.14
As shown by Table 5, a twin screw extruder may be used to form foam
materials having small and uniform cells. However, a single
extruder provides a shorter distance in which to melt and mix the
solution and reduce its temperature to the exit temperature.
Examples 34-41
Examples 34-41 illustrate coextrusion of foamed materials with
polymeric outer layers or "skins." The foam cores for examples 34
to 41 were processed using the tandem extrusion method previously
described with the operating conditions shown in Table 6. For
examples 37-39, extruder 20 was operated at 15 rpm. The polymeric
skins in examples 37 and 38 comprised a tacky polymeric material
available as HL-2647 from HB Fuller, St. Paul, Minn. The polymeric
skin precursor material was processed using a 32 mm diameter single
screw extruder, having 3 zones and a length to diameter ratio of
24:1, available from Killion, Pawcatuck, Conn. For examples 34-36,
the temperatures in Zones 1-3 of this extruder were set at
121.degree. C., 121.degree. C., and 121.degree. C. For example 37,
the temperatures in Zones 1-3 of this extruder were set at
121.degree. C., 160.degree. C., and 154.degree. C. For examples 38,
the temperatures in Zones 1-3 of this extruder were set at
121.degree. C., 160.degree. C., and 149.degree. C. For examples
39-41, the temperatures in Zones 1-3 of this extruder were set at
138.degree. C., 215.6.degree. C., and 204.degree. C. For examples
34 to 41, respectively, the Killion extruder was operated at 8.7,
74, 40, 25.3, 50.3, 0, 81, 15 rpm, which varied the skin thickness.
Coextruded articles were made using a two layer tubular die having
an outlet diameter of 4.8 mm. The foamable melt solution was fed to
the center portion of the two layer die and the polymeric
(unfoamed) skin material was fed to the outer portion of the die
resulting in a two layer tubular structure exiting the die. The
extruded tubes were cut and collected.
Foam cell size, and polydispersity for examples 34-41 are shown in
Table 6.
TABLE 6 Core CO.sub.2 Skin Core cell size Ex. Com- Flowrate
Flowrate Concentration T.sub.exit T.sub.exit -Tg P.sub.exit Com-
Flowrate Ave. Std. Dev. p position (kg/hr) (kg/hr) (wt %) (.degree.
C.) (.degree. C.) (Mpa) position (kg/hr) (.mu.m) (.mu.m) P 34 100%
20.4 1.25 5.8 111 +6 21.8 100% 2.4 35 8 1.04 PS615 HL-2647 35 100%
20.4 1.25 5.8 111 +6 11.0 100% 4.5 42 10 1.04 PS615 HL-2647 36 100%
20.4 1.25 5.8 111 +6 20.4 100% 1.4 35 8 1.05 PS615 HL-2647 37 100%
9.6 1.04 9.8 96 -9 12.1 100% 5.6 22 14 1.18 PS615 PS615 38 100% 9.6
1.04 9.8 96 -9 23.8 100% 5.2 20 12 1.32 PS615 PS615 39 100% 9.6
1.04 9.8 96 -9 18.4 none 0 19 4 1.04 PS615 40 100% 10 1.25 11.0 107
+2 23.8 100% 4.6 40 11 1.08 PS615 PE 6806 41 100% 10 1.25 11.0 107
+2 15.1 100% 3.1 17 4 1.04 PS615 PE 6806
As shown by Table 6, coextruded articles having small and uniform
cells can be made using a process of the invention.
* * * * *